The present invention relates to a paste type positive electrode for alkaline storage batteries, as well as to a nickel-metal hydride storage battery.
Secondary batteries are mounted on most of portable apparatuses, such as cellular phones and laptop computers. Under such circumstances, development of a secondary battery having the higher capacity has strongly been desired. The positive electrode has been improved as discussed below to attain the higher capacity of alkaline storage batteries.
Both a sintered type positive electrode and a paste type positive electrode are applicable for the alkaline storage battery. The sintered type positive electrode has a substrate having pores of approximately 10 μm in diameter. The substrate is obtained by sintering a nickel powder and a core material (e.g. perforated metal sheet, etc), and has a small porosity of approximately 80%. This substrate is accordingly filled with a relatively small quantity of the active material. The paste type positive electrode, on the other hand, has a foamed nickel substrate having pores of approximately 500 μm, where the pore is communicating each other and arranged in a three-dimensional manner. The foamed nickel substrate has a large porosity of approximately 95%. This substrate is accordingly filled with a relatively large quantity of the active material. Namely the paste type positive electrode has a higher capacity.
The electrical conductivity of nickel hydroxide, which is the active material of the paste type positive electrode, varies with a variation in oxidation number of nickel: that is, high conductivity for the large oxidation number and low conductivity for the small oxidation number. The oxidation of nickel hydroxide in charging process of a battery thus proceeds smoothly, but the reduction in discharging process does not proceed smoothly, due to the lowered electrical conductivity in the terminal stage of discharging process. This causes an insufficient discharge. A conductive agent, such as a cobalt compound, is added to the active material, with a view to enhancing the electrical conductivity in the positive electrode and ensuring the sufficient discharge.
In the case where cobalt hydroxide is added to the active material, the first charging cycle after the manufacture of the battery causes cobalt oxyhydroxide having a good conductivity to deposit on the surface of nickel hydroxide as the active material. This ensures the favorable conductive networks (Japanese Laid-Open Patent Sho 61-74261). Cobalt oxyhydroxide is stable in a standard voltage range of the battery and keeps the conductive networks.
In an alkaline storage battery, the negative electrode generally has a greater capacity than the capacity of the positive electrode. The residual non-charged capacity of the negative electrode under the condition where the positive electrode is completely charged is referred to as the charge reservoir, and the residual charged capacity of the negative electrode under the condition where the positive electrode is completely discharged referred to as the discharge reservoir.
When the battery is excessively charged, the reaction as defined below occurs at the positive electrode to produce oxygen:OH−→1/2H2O+1/4O2+e−Oxygen reacts with the hydrogen absorbed in the negative electrode and is consumed:MH (metal hydride)+1/4O2→M (alloy)+1/2H2OM+H2O+e−→MH+OH−The hydrogen storage alloy of the negative electrode hardly absorbs hydrogen in the terminal stage of charging process of a battery. The presence of the alloy that has not yet absorbed hydrogen as the charge reservoir effectively depresses the generation of gaseous hydrogen. This enables the battery to be sealed.
The following describes the discharge reservoir in the general paste type positive electrode including nickel, which is obtained by adding cobalt hydroxide as a conductive agent to nickel hydroxide functioning as the active material. The initial charging of the battery having this positive electrode changes cobalt hydroxide to cobalt oxyhydroxide. The electrical quantity stored in the negative electrode while this process becomes part of the discharge reservoir.
The capacity of nickel hydroxide is 289 mAh/g, and the capacity of cobalt hydroxide is 288 mAh/g. In the case where cobalt hydroxide in an amount of 10% by weight of nickel hydroxide is used, therefore, the discharge reservoir obtained is approximately one tenth of the capacity of the positive electrode.
The oxidation number of nickel in nickel hydroxide is initially 2 but rises to about 3.2 by charging of the battery, so that nickel hydroxide is changed to nickel oxyhydroxide. The discharge of the battery is concluded when the oxidation number of nickel decreases to about 2.2. The non-discharged nickel oxyhydroxide thus remains to give the discharge reservoir of approximately two tenths of the capacity of the positive electrode. The nickel-metal hydride storage battery accordingly has the total discharge reservoir of approximately three tenths of the capacity of the positive electrode.
The adequate quantity of the discharge reservoir is at most about one tenth of the capacity of the positive electrode. Namely the capacity corresponding to approximately two tenths of the capacity of the positive electrode are excessive in the negative electrode. In other words, the prior art battery includes a specific quantity of the hydrogen storage alloy that does not contribute to charging and discharging. Regulating the quantity of the discharge reservoir to the appropriate level desirably reduces the required quantity of the expensive hydrogen storage alloy and gives a battery of high energy density at a low manufacturing cost.
From these viewpoints, the positive electrode of the battery disclosed in Japanese Laid-Open Patent Sho 60-254564 includes nickel hydroxide, cobalt, and nickel oxyhydroxide required for oxidation of cobalt. This proposed battery has the reduced discharge reservoir accompanied with oxidation of cobalt. The positive electrodes of the batteries disclosed in Japanese Laid-Open Patent Hei 4-26058 and Hei 8-148145 include particulate nickel hydroxide with cobalt oxyhydroxide carried thereon.
The battery disclosed in Japanese Laid-Open Patent Hei 11-219701 seems to attain the greatest effect of reducing the discharge reservoir, among the various prior art batteries. The positive electrode of this proposed battery includes a first active material, which comprises particulate nickel hydroxide with cobalt oxyhydroxide carried thereon, and a second active material, which comprises particulate nickel oxyhydroxide with cobalt oxyhydroxide carried thereon. The weight ratio of the first active material to the second active material ranges from 90/10 to 60/40.
In the positive electrode of the battery disclosed in Japanese Laid-Open Patent Hei 11-219701, however, the oxidation number of nickel in the nickel oxyhydroxide of the second active material is not specified. The quantity of the discharge reservoir in the negative electrode depends upon not only the weight ratio of the first active material to the second active material but the oxidation number of nickel in the nickel oxyhydroxide of the second active material. Namely the appropriate quantity of the discharge reservoir is unknown in the battery disclosed in Japanese Laid-Open Patent Hei 11-219701.